<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <link rel="stylesheet" href="https://unpkg.com/element-ui/lib/theme-chalk/index.css">
  <style>
      *{
          margin:0;
          padding:0;
      }
      html,body,#app{
          height:100%;
          background:pink;
      }
      #app{
          display: flex;
          flex-direction: column;
      }
      .el-header{
          background:blue;
      }
      .el-main{
          background:green;
          flex:1;
      }
  </style>
</head>

<body>
  <div id="app">
    <el-container>
        <el-header height="100px">Header</el-header>
        <el-main>Main</el-main>
    </el-container>
  </div>
  <script src="https://cdn.jsdelivr.net/npm/vue@2.6.10/dist/vue.js"></script>
  <script src="https://unpkg.com/element-ui@2.9.1/lib/index.js"></script>
  <script>
    var vm = new Vue({
      el: '#app',
      data: {},
      methods: {}
    });
  </script>
</body>

</html>